A hydraulic servovalve is a servo with a device (either flapper nozzle or jet pipe) used to position the servo. When servovalves are controlled through an electrical signal they are called electrohydraulic servovalves. Servovalves are normally used when accurate position control is required and this position control may be achieved through a closed loop control system, consisting of command sensor, feedback sensor, digital or analogue controller, and the servovalve.
Flapper nozzle systems for use in servovalves are well known. Flapper position is controlled by the electromagnetic torque motor and the torque developed by the torque motor is proportional to the applied current, with currents generally being in the milliamp range. A torque motor consists of two permanent magnets with a coil winding attached to a magnetically permeable armature. The armature is part of the flapper piece. When a current is applied to the coils, magnetic flux acting on the ends of the armature is developed. The direction of the magnetic flux (force) depends on the direction of the current. The magnetic flux will cause the armature tips to be attracted to the ends of the permanent magnets (current direction determines which magnetic pole is attracting and which one is repelling). This magnetic force creates an applied torque on the flapper assembly, which is proportional to the applied current. In the absence of any other forces, the magnetic force would cause the armature to contact the permanent magnet and effectively lock in this position. However, other forces are acting on the nozzle, such that flapper position is determined through a torque balance consisting of magnetic flux (force), hydraulic flow forces through each nozzle, friction on the flapper hinge point, and any spring (wire) connecting the flapper to the spool (which is almost always used in servovalves to improve performance and stability).
As the applied current is increased, the armature and flapper will rotate. As the flapper moves closer to one nozzle, the flow area through this nozzle is decreased while the flow area through the other nozzle increases.
Servovalves can be used to control hydraulic actuators or hydraulic motors. When a servoactuator is used to control an actuator, the servovalve and actuator combination are often referred to as a servoactuator. The main advantage of a servovalve is that a low power electrical signal can be used to accurately position an actuator or motor. The disadvantage is their complexity and the resulting costs of components consisting of many detail parts manufactured to very tight tolerances. Therefore, servovalves are generally only used when accurate position (or rate) control is required.